1. Field of the Invention
The present invention relates to spectrophotometry. In particular, the present invention relates to a rugged, compact spectrophotometry system that includes a spectrophotometer, a multiplexer, and an on-board computer.
2. Discussion of Background
Spectrophotometric techniques based on light emission, absorption or scattering processes are widely used for qualitative and quantitative analyses. A typical spectrophotometry system includes several basic components: a light source, a collimator, a light-dispersing mechanism such as a prism or diffraction grating, and a detector. In absorption spectroscopy, light from the source passes through a sample and is focused onto the detector, which produces an output signal proportional to the absorbance of the sample. The absorbance or absorption spectrum of the test sample is compared with the spectra of other samples containing known concentrations of various substances in order to determine the quantitative amounts of these substances present in the test sample.
Recent developments in fiber optics, coupled with the availability of multichannel array-type spectrometers and multiplexing technology, have generated renewed interest in the use of remote spectroscopic techniques for in-line monitoring and process control, environmental monitoring, and medical applications. Signal transmission via optical fibers allows for the placement of sensitive equipment in central locations, making remote sensing a particularly attractive choice for monitoring processes that take place in harsh industrial process environments. In the environmental field, remote sensing techniques are used for in situ measurements of fluids in wells, boreholes, storage and process tanks, etc. Applications include monitoring groundwater flow, studying the migration of subsurface contaminants, evaluating the progress of remediation operations, and detecting toxic or explosive substances.
Optical analysis techniques also improve the quality of the data. Data obtained from a sample are not always truly representative of the source of that sample, since the mere act of taking the sample can alter its properties; frequently, removing a sample can perturb the source as well. Optical techniques can frequently be implemented without the need to take samples for laboratory analysis elsewhere; therefore, data from optical analyses can be more reliable than data obtained using other analytical techniques.
Multiplexing--using a single instrument to measure and analyze signals from a plurality of sources--furthers the efficient use of complex and expensive instrumentation. Optical fibers connect a plurality of probes to a multiplexer, which in turn is connected to a single measuring instrument such as a spectrophotometer. Measurements are made by switching between probes, so that each probe in turn is connected to the instrument. The principal advantage of multiplexing is that a single light source and a single detector can be used for measuring the outputs of many probes, even probes at widely-separated locations.
In U.S. Pat. No. 5,131,746, O'Rourke, et al. describe an on-line process control monitoring system that makes use of a plurality of fiber optic probes, each probe at a different process location. The system includes a light source, optical fibers for carrying light to and from the probes, a multiplexer for switching light from the source from one probe to a next in series, a spectrophotometer, and a computer programmed to analyze the spectra. Standard and reference cells may be included for data validation and error checking. Alternatively, self-referencing measurements utilizing the method described in U.S. Pat. No. 5,298,428 can be implemented. Here, two successive absorption spectra of a sample containing a photoreactive substance are compared to determine the concentration of the substance. For non-photoreactive substances, a photoreactive dye can be added to the sample to make a mixture with photoreactive properties unique to the mixture. The disclosures of U.S. Pat. Nos. 5,131,746 and 5,298,428 are incorporated herein by reference.
A wide variety of fiber optic probes are available for use with spectrophotometry systems. By way of example, U.S. Pat. No. 5,168,367 describes a variable path length probe for spectrophotometric measurements of fluids in situ. For Raman-type measurements of scattered light, probes designed for improved light coupling efficiency such as the probe described in U.S. Pat. No. 5,402,508, the disclosure of which is incorporated herein by reference, are useful. The probe includes a housing with a transparent window across its tip for protecting the transmitting and receiving fibers. The endfaces of the fibers are slanted, resulting in improved light coupling efficiency between the transmitting and receiving fibers. Other suitable probes include those disclosed in the following commonly-assigned, co-pending applications, the disclosures of which are incorporated herein by reference: Ser. No. 08/676,432, filed Jul. 8, 1996 (Fiber Optic Probe); Ser. No. 09/031,527, filed Feb. 27, 1998 (Retro-Reflection Probe With Collimating Lens Assembly); Ser. No. 09/032,073, filed Feb. 27, 1998 (Fiber Optic Probe for Attenuated Total Internal Reflection Spectrophotometry); Ser. No. 09/031,521, filed Feb. 27, 1998 (Fiber Optic Raman Probe and Coupler Assembly); Ser. No. 60,076,140, filed Feb. 27, 1998 (Fiber Optic Probe System for Spectrophotometric Analyses).
Despite the advantages of both spectrophotometry and multiplexing technology for sample analysis and process control, widespread use of these technologies outside the laboratory has been limited by the twin problems of cost-effectiveness and reliability. A wide variety of spectrophotometers are available; however, most of these instruments are relatively complex and delicate, expensive, and require skilled operators to ensure the accuracy and reliability of the data. Without adequate cooling systems, temperature changes during operation of many spectrophotometers may lead to warping, which decreases the reliability of the measurements. Small, light-weight units are particularly susceptible to heating during extended use, which renders measurements made with these units inherently unreliable.
Optical multiplexers have a plurality of closely-spaced fiber terminals or optical probes arranged in linear or rectangular arrays. To make a series of measurements using such a device, an optical fiber leading to the detector is precisely aligned with each selected terminal (or probe) in turn by a stepper motor or similar device. Alternatively, the array itself can be moved so as to align different terminals with the detector. To ensure reproducible, dependable measurements, the individual fiber terminals in the array need to be very precisely positioned with respect to each other, requiring each position in the array to be machined to extremely close tolerances. Manufacturing these types of multiplexers is a difficult, labor-intensive and time-consuming process which requires costly precision machining equipment. Even though a number of multiplexed spectrophotometry systems are available, the cost of a rugged, reliable system that is capable of providing useful data is simply too high for many potential users.
There is a need for a simple, rugged multiplexed spectrophotometry system that can be readily adapted for laboratory, industrial process and field use, and that can be manufactured and assembled without costly, high-precision equipment. Such a system would include a compact spectrophotometer and a multiplexer with a plurality of precisely-alignable connectors for optical probes.